Vinyl alcohol-based polymers (hereinafter, a vinyl alcohol-based polymer may be abbreviated as “PVA”) represented by polyvinyl alcohol are one of the few crystalline water soluble polymers. PVA is used as a raw material for a synthetic fiber, vinylon, and the like utilizing strength properties thereof. The PVA is also utilized for paper processing agents, textile sizing agent, dispersants, adhesives, films, and the like utilizing the excellent film formation ability, surfactant ability, hydrogen bond formation ability, and the like. Further, since the PVA has excellent interfacial properties and strength properties, it is also utilized for stabilizers for emulsion, various binders, and the like.
The PVA is also subject to higher functionality by improving specific performances by controlling the crystallinity or introducing functional groups, and so-called modified PVA is variously developed as well.
Among them, PVA having a polyoxyalkylene (hereinafter, may be abbreviated as “POA”) group is variously developed. For example, Patent Document 1 discloses a method of synthesizing PVA having a POA group in which saponified is a copolymer of a vinyl ester and POA having an allyl group at an end, a methacrylic ester having a POA group, or a methacrylamide group containing compound having a POA group bonded to a nitrogen atom. After development of such synthesis method, PVA having a POA group has come to be utilized for a variety of applications.
The PVA is utilized in paper processing applications for improvement in quality of printed matter. The PVA is utilized in the paper processing applications for binders for fillers in ink jet recording materials, surface sizing agents for general paper, undersizing agents for art paper and coated paper, dispersants for fluorescent dyes, and the like.
Recently, ink jet printers have been increasingly utilized, and so ink jet recording materials are used for color proof in commercial printing, design image output in the design field, drafts for overhead projectors, and the like. The properties demanded for an ink jet recording material in these applications may include that an ink receiving layer surface is high gloss, that the transparency of the ink receiving layer is high, that the image density is high, that the color reproducibility is good, that the ink absorbency is high, that the dot reproducibility is good, and the like.
To the above demands for an ink jet recording material, there is a known recording material equipped with a porous layer composed of inorganic microparticles and a hydrophilic binder as an ink receiving layer (refer to Patent Documents 2 and 3). This recording material realizes both high ink absorbency and waterproofness by an ink absorbency mechanism utilizing the capillary action. However, in order to form capillaries in the ink receiving layer, it is usually required to contain a large amount of microparticles relative to the binder. In this case, the ratio of the binder to the microparticles becomes low, and the ink receiving layer becomes high in rigidity and becomes hard. Therefore, in forming an ink receiving layer by coating a coating liquid composed of the inorganic microparticles and the hydrophilic binder on a substrate, there used to be a problem that cracks easily occur in the ink receiving layer due to the occurrence of internal stress in the stage of drying a coating film or incorporation of minute foreign substances.
For such problem of cracks in the coating film, a method of preventing cracks by thickening the coating film before drying the coating film is considered. As one of such methods, there is proposed a method comprising cooling a coating film to not more than 20° C. after coating using a coating liquid in which boric acid is added as a curing agent to polyvinyl alcohol having a high degree of polymerization (refer to Patent Document 4). In this method, as the coating film is cooled to not more than 20° C., a strong three dimensional structure is formed in the coating film due to the interaction between the polyvinyl alcohol and the boric acid, and as a result, it is supposed to allow prevention of cracks. However, since this method requires the temperature of the coating film to be lowered once after coating, there used to be a problem that the energy loss is large. In addition, since the temperature of the coating film is lowered once after coating, there also used to be a problem that it takes time to form a coating film and it is not possible to increase the production rate. Further, since this method uses polyvinyl alcohol having a high degree of polymerization, the viscosity of the coating liquid at room temperature used to be high. Accordingly, in respect of the handling properties, it used to be required to keep the concentration of the coating liquid low. In a case that the concentration of the coating liquid is low, it used to take time to dry the coating film and the production rate used to decrease even more.
As a method of controlling the viscosity of a coating liquid containing PVA, there is also considered, other than the method comprising adding an additive having reactivity to PVA as described above, a method comprising using modified PVA. There are some, although not so many, reported attempts to control behavior of the solution viscosity of PVA relative to the temperature changes by modifying PVA. Patent Document 5 describes a block copolymer and a graft copolymer composed of a polyvinyl alcohol component and a polyalkenyl ether component exhibiting a cloud point in an aqueous solution as PVA exhibiting thermal responsivity in an aqueous solution. However, in a case of a block copolymer of polyvinyl alcohol and poly (2-methoxyethyl vinyl ether), there used to be a problem that the temperature dependency of aqueous solution viscosity of the block copolymer in a temperature range of from 40° C. to 90° C. is less compared with a case of polyvinyl alcohol. There is also a known graft copolymer having polyvinyl alcohol as a stem component and poly(2-methoxyethyl vinyl ether) as a branch component (refer to Patent Document 6). However, there used to be a problem that the aqueous solution viscosity of the graft polymer increases around the temperature where an aqueous solution of each polymer used for the stem component and the branch component exhibits a cloud point and the aqueous solution becomes turbid.
Patent Document 7 describes PVA having a polyoxyalkylene group. The temperature-sensitive thickening properties are supposed to be given to the PVA by adjusting the polyoxyalkylene group content or the number of repeating units in the polyoxyalkylene group in the PVA. Then, it is described that the temperature-sensitive thickening properties of the PVA is applied to enable production of an ink jet recording material with fewer cracks.
However, the PVA described in Patent Document 7 used to have a polyoxyalkylene group composed mainly of a polyoxypropylene group. Although such PVA having a polyoxyalkylene group composed of a single oxyalkylene unit has the temperature-sensitive thickening properties, there used to be a case that the properties do not occur when the PVA concentration in the solution is low. Therefore, with respect to the cracks in a coating film due to drying, the improvement effect has been insufficient although there is a little. Patent Document 8 describes PVA having a polyoxybutylene group, while the PVA has high viscosity around room temperature and handling properties thereof has been a drawback.
PVA is also utilized for release paper. The release paper has a release performance on a surface thereof and is used as a substrate to obtain adhesive labels, adhesive tapes, industrial adhesive paper, and the like and also as mold release paper and the like. The release paper is usually obtained by coating oil varnish containing a silicone resin, a solvent (toluene and the like), and the like capable of forming a water repellent and oil repellent coating film on paper to form a release layer. However, paper is porous between pulp fibers and the like, and thus has high permeability and gas permeability. Therefore, when oil varnish is coated directly on a paper surface, the oil varnish turned out to permeate in the paper and release paper having a sufficient release performance used not to be obtained. Further, since the oil varnish permeates in the paper, which increases the amount of coating and is uneconomical.
For such problem, a method of utilizing the excellent film formation ability and grease resistance is generally employed. That is, a method comprising coating oil varnish on release paper base paper prepared by coating an aqueous PVA solution as a coating agent (filling agent) in advance on a paper surface. However, usual PVA is poor in waterproofness, although excellent in grease resistance for release layer paint and organic solvents contained therein, and there used to be a problem that PVA elutes due to humidification during the adhesive layer process for blocking.
For the purpose of improvement of the waterproofness of PVA, release paper base paper is proposed that uses modified PVA containing α-olefin unit having a carbon number of 4 or less (refer to Patent Document 9). However, even more improvement in the performances has been sought.
For the purpose of such performance improvement, release paper base paper is proposed that uses modified PVA containing a polyoxyalkylene group in side chain (refer to Patent Document 10). However, an aqueous solution of the modified PVA is high in viscosity even around room temperature and handling properties thereof have been a drawback.
PVA is also utilized for greaseproof paper. The greaseproof paper is defined as “1) Generic term for paper having grease resistance. 2) Paper or board having extremely great resistance to permeation of grease or fat.” in “Paper, board and pulp—Vocabulary.” of JIS P 0001: 1998.
In packaging materials for foods, greaseproof paper having grease resistance is widely used. Among all, greaseproof paper is used for foods containing a lot of oil and fat components, such as chocolate, pizza, and donut, not to allow oil to permeate the packaging material. When the oil and fat components contained in foods permeate the packaging material, various types of inconvenience occur. For example, oil permeates even through a surface where the food is not in contact to create oil stain, which detracts from the appearance and reduces the commodity value, and the oil stain darkens a printed area resulting in causing characters not to be legible or decreasing the OCR qualifications of a bar code, a QR code (registered trademark), or the like. There are also problems of oil transfer to clothing for staining and the like, so that greaseproof paper is used for an area in contact with foods.
Conventionally, in order to cause greaseproof paper to develop grease resistance, a fluorine-based compound, particularly a perfluoro fluorine-based compound has been used as a grease resistant agent. However, since a perfluoro fluorine-based compound produces perfluorooctanoic acid or perfluorosulfonic acid by heat treatment, there is a concern for safety. Therefore, various fluorine-based compounds with enhanced safety are developed, however realizing both grease resistance and safety still has been insufficient.
In addition, it is known that PVA may be used as a non-fluorine-based grease resistant agent. PVA is a hydrophilic resin and forms a strong film. Therefore, PVA prevents permeation of oil and is excellent in grease resistance. As a method of giving grease resistance to a paper substrate by PVA, known are, for example, a method comprising forming a coating layer containing PVA and shellac on at least one surface of a substrate sheet (refer to Patent Document 11), a method comprising forming a coating layer containing PVA cured using an isocyanate compound on at least one surface of a substrate sheet (refer to Patent Document 12), and the like.
However, in the above methods, while it is possible to improve the waterproofness, it is not possible to improve the grease resistance sufficiently. In addition, in the above methods, PVA easily permeates paper, so that pinholes easily occur. Therefore, it is required to increase the amount of coating a coating agent containing PVA and the drying load becomes too large, which used to be a problem of extremely reducing the productivity.
PVA is also utilized for a film. Then, PVA having a POA group is also utilized fora film. For example, Patent Document 13 proposes a resin for melt molding capable of being used as a raw material for a film, Patent Document 14 proposes a film for packaging alkaline substances, and Patent Document 15 proposes a flexible waterproof film in which heat treatment of the film is devised.
In above Patent Documents 1, 13 through 15, to obtain a polymer or a film having excellent water solubility at low temperatures, PVA having an oxyethylene group or a polyoxypropylene group as a POA group is used. However, there used to be a problem that a film containing such PVA has decreased film strength.